I did a total of 3x drain and fills with DEXRON VI within a few k miles of getting my 2001 3.0i. Had ~160k miles on the original fluid when I started. No problems, and very glad I did it. Smoother, faster shifting. Did it carefully, which I posted on this site back when I did it.

On the DEXRON VI vs. III, I believe the low temp viscosity of the VI is lower, and you can pretty easily see/feel this when you pour it in. But the VI is more stable, retaining the viscosity both with temp increase and with wear. So as the fluid actually operates in your AT, at temp, and after a few k miles of use, the VI will have slightly higher viscosity, which is preferred.

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2001 X5 3.0i, 203k miles, AT, owned since 2014